1. Field of the Invention
Cladding-pumped fiber structures such as lasers and amplifiers.
2. Description of Related Art
U.S. Pat. No. 5,418,880, May 23, 1995 is representative of the relevant art. The cladding pumped laser is an answer to the need for a high-power fiber laser. It relies on a multi-mode (pump) cladding for accepting and transferring pump energy to an active core along the length of the device through the core-clad interface. Energy is guided within the pump cladding by a second cladding of lower index of refraction. The configuration is likely to be used in fiber amplifiers as well as in fiber lasers.
The structure was immediately recognized as promising, and attention was soon directed to design optimization. Communication fiber had not been designed for the purpose. The need for more effective introduction of pump energy into the cladding, e.g. from a laser array, led to use of claddings of enlarged cross-section. The usual, circularly symmetrical, design used for communication fiber was no assurance of effective clad-to-core energy transfer, as it produced pump modes of helical paths which did not cross the clad-to-core interface. Reconsideration of fundamental parameters such as index profile were restudied as well. See, Optics Commun., vol. 99, pp. 331-335 (1993).
U.S. Pat. No. 4,815,079, Mar. 21, 1989, improves coupling efficiency by either of two approaches. In the first, the core is placed off-center within an otherwise circularly symmetric structure. The second uses a high aspect-ratio rectangular pump cladding enclosing a centrally-located core. Both increase incidence of interface crossings by pump modes, and improve transfer efficiency. In common with other suggested structures, a second (outer) clad of lower index than that of the first (pump) clad, guides pump energy within the multi-mode pump clad.
U.S. Pat. No. 5,418,880, largely directed to structures for effectively coupling the output of a diode array into the pump cladding, also considers non-circular inner cladding designs. It proposes any of various polygonally shaped pump claddings--three-, four- and six-sided, some regular, some irregular--within outer claddings of low-index organic polymer. The design objective is uniform distribution of pump energy across the fiber cross section, assuring effective transfer.
Asymmetric structures are difficult to align and to splice to conventional transmission fiber. Rectangular and other irregular polygonal cross-sections also pose a problem. By all accounts, regular polygonal pump claddings deviate least from circularly symmetric structures and are preferred.
The structures of these patents do improve clad-to-core coupling efficiency, but fabrication is problematic. Manufacture of off-center preforms and procedures for drawing from such preforms do not use established technology. Fabrication of polygonal preforms from conventional round preforms by grinding and polishing is costly. Strength may be compromised--incidence of breakage under stress is increased by jagged surface features retained from machining.